You’re having a bad day. You snap at your spouse, act short with your colleagues, and cut off other drivers on your commute home. Are you the victim of a bad mood? Or is your problem that your brain is infected with behavior-modifying parasites?

It’s a disgusting prospect, but a brain infection might well be the cause.

There’s something innately repellent about parasites – organisms that invade their hosts and feast upon their bodies from within. But in the gallery of biological horrors a special place has to be reserved for that bizarre and horrid class of parasites which hijack not only their hosts’ bodies but their brains as well, causing them to engage in behavior that suits the purposes of the invading organism.

Thousands of such parasites are known to science, most of them fortunately confined to the insect world. There’sCordyceps, a fungus that germinates within its host and, when the moment comes, commands it to climb to a high prospect such as a tall blade of grass. Once there, the insect clamps itself in place and dies. Gradually, from within its corpse, the fruiting body of the fungus emerges, ultimately to spread its spores from the great height its victim’s behavior has provided. As this video clip shows, a huge range of insect are attacked by Cordyceps, in a highly specific way: each species of the fungus can only attack one or two species of host. Over time Cordyceps has evolved a breathtaking variety of forms, many of both beautiful and spine-tinglingly creepy.

Hairworms affect grasshoppers in a different way. They cause the creatures to jump into water, where they drown, releasing the hairworms’ offspring to another round of infection. The trematode parasite Microphallus causes freshwater snails to climb to exposed places where they can more readily be eaten.

The protozoan Toxoplasma gondii causes perhaps the most subtle behavioral change of all: when it infects rats, it selectively attacks the animals’ amygdala so that it no longer fears one of its main predators, the cat. In fact, it is actually attracted to the scent of cat urine. Fascinatingly, its fear system seems to be otherwise unaffected: it still dreads open spaces, and it can learn to avoid electric shocks. The only fear that is canceled out is the one that’s most likely to cause the rat to be eaten by the parasite’s favorite host. And so the life cycle continues.

Toxoplasma is fascinating especially because, unlike most behavior-altering parasites, it affects not insects but mammals, which have far more sophisticated nervous systems. And it even seems to affect humans. No, it doesn’t make us crave cat pee, but it does meddle with our circuitry in a way that may be related to amygdala dysfunction. Those infected are more likely to suffer from schizophrenia and neuroticism and to engage in dangerous risk-taking: A recent study of Czech army conscripts found that those infected by Toxoplasma are six times more likely to be involved in a car crash.

Okay, so Toxoplasma infection can alter our brains for the worse. But how likely is a given individual to be affected – something so weird must be quite rare, right? Unfortunately, quite the opposite. Between 20 and 60 percent of any given population is infected, and once you’ve got it, it’s for life.

It’s hard to see what Toxoplasma gets out of all the suffering it imposes on humanity – probably nothing, since humans are rarely eaten by cats, except occasionally by big ones in Africa or India. Most likely, we’re just collateral damage, suffering because the invading Toxoplasma mistakes us for a more cat-attractive host. So in a sense, we’re lucky: as unpleasant as it may be to suffer from a brain parasite infection, at least you’re not going to wind up getting pounced on and eaten.

3 Comments

I didn’t see that one but, from what I read — SPOILER ALERT! — it’s about trees which release a pheromone or somesuch that causes people to kill themselves. The closest movie example I can think of would be the title monsters in Alien, which lay their eggs inside living human hosts, which hatch and eat them from inside and then pop out — they don’t alter the host’s behavior, though, I don’t think. It’s been a long time…

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